iOS8 Update

Ok, so right now I am in the process of updating my iPad from a late version of ios 7 to ios 8.1.3 - Anf I have some questions/concerns.

*When you are on the screen to update, it shows how much time is left. It's been changing a lot like earlier it went from about 8 hours to 28 hours, etc. Does that mean anything? And about how many hours should it take?

*2nd off, I read on some other posts that a lady updated her iPad 2 to the ios 8 software, and it messed up her iPad. That's my concern... I have an iPad air and I was just wondering if the iPads crash a lot during updates.

Thanks 🙂